Influences of Cu alloying on precipitation, austenitic reversion and mechanical properties of NiAl-strengthened medium-Mn steels

We have investigated the multiple influences of Cu addition into the NiAl-strengthened medium Mn steels (MMS) on the precipitation and austenitic reversion during the different ageing processes and the resultant mechanical properties, and conclude that such an alloying is the exceptional approach of simultaneously improving yield strength, work hardening sustainability and plasticity all together due to the following reasons. First, it promotes the precipitation and growth of NiAl for greater precipitation hardening because the prior Cu-rich precipitation provides the dispersed nucleation sites for NiAl in company with Ni/Mn enrichment at the interface of Cu and martensite. Next, such an enrichment also accelerates the nucleation and growth of austenite on the Cu-NiAl coprecipitates during the ageing, leading to larger fraction of austenite grains retained with coarser size than those in the non-Cu-alloyed counterpart. Particularly, the presence of Cu nanoparticles in the reverted austenite may result in not only strengtheneing but also the increase of Mn content in the interiof of austenite by retarding the migrating phase interface, both improving mechanical stability of retained austenite (RA). Consequently, the Cu alloying significantly increases yield strength, work hardening sustainability and ductility simutaeously due to greater precipitation strengthening and more durable austenite-to-martensite transformation during deformation.